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NL Auction Rooms in North Finchley sold the three-piece garniture made of the unique Derbyshire fluorspar on September 3. Estimated at £500-1000, the set sold for £18,500.

They were consigned by a private vendor who had inherited them. Catalogued as possibly George III period, they may well be mid-19th century French.

Blue John is a semi-precious mineral, a form of fluorite with bands of a purple-blue or yellowish colour found at the Blue John Cavern and Treak Cliff Cavern at Castleton in Derbyshire. During the 19th century it was mined for its ornamental value, and mining still continues on a small scale.

The highlight was a £21,000 (plus 20% buyer's premium) late 18th century urn with ormolu mounts, which measured 16in (41cm) high, and was attributed to the workshop of Birmingham industrialist Matthew Boulton.